Do global inversion systems systematically underestimate the seasonal amplitude of net ecosystem CO2 flux?

The Atmospheric Carbon and Transport (ACT) America mission gathered an unparalleled set of airborne greenhouse gas (GHG) data suitable to evaluating regional, seasonal ecosystem flux estimates from the central and eastern North America. We present the results from three independent evaluations of global atmospheric inversion systems using these data. Each of these studies compare observed and simulated regional CO2 mole fractions to address this question, but using different approaches to quantify atmospheric transport, and to isolate continental biogenic CO2 fluxes. One study compares the posterior CO2 mole fractions from ten global inversion systems applied in OCO-2 v9 MIP to the ACT-America airborne observations, focusing on vertical and cross-frontal CO2 differences. A second study evaluates the posterior CO2 fluxes from nine global inversion systems applied in OCO-2 v9 MIP using a Lagrangian particle dispersion model to map the fluxes into mole fraction space and compare to ACT-America airborne data. A third study propagates posterior CO2 flux estimates of one global inversion system forward for summertime using an independent numerical weather model ensemble and compares these mole fractions to ACT-America and NOAA tall tower CO2 measurements. Each study suggests that current global atmospheric inversion systems may underestimate the amplitude of the seasonal cycle of net ecosystem-atmosphere exchange (NEE) of CO2 across the central and eastern regions of North America. The cause of this apparent bias is not clear, nor are the implications for the net annual biogenic flux of CO2 from North America or nearby regions.